1. Field of the Invention
The present invention relates to a device and a method for driving a display panel.
2. Description of the Related Art
In a conventional display panel employing organic electroluminescence elements (referred to simply as EL elements hereinafter), the organic EL elements are arranged in a matrix and are illuminated by using a driving device which includes an anode driver and a cathode driver (for example, Japanese Patent Kokai No. 2000-40074, No. 2001-350431, and No. H06-301355). The drivers are often built in the form of a single chip IC so that the driving device can be made small.
FIG. 1 shows a conventional organic EL display panel having a so-called simple matrix configuration with a plurality of row electrode lines and a plurality of column electrode lines, and also shows the configuration of a driving device for the display panel.
In FIG. 1, organic EL elements ELD are arranged in a matrix (n rows×m columns) in an organic EL display panel 1.
Each of the organic EL elements consists of an anode electrode, a cathode electrode, and an organic EL light emitting layer sandwiched between the anode electrode and the cathode electrode, and has rectifying properties like ordinary diodes. Also, as shown in FIG. 1, since each of the organic EL elements ELD has a parasitic capacitance C in the organic EL light emitting layer, the parasitic capacitor C is indicated as an element equivalently connected in parallel to each of the organic EL elements ELD.
The anode electrodes of the organic EL elements ELD are connected to an anode driver IC 2 through the corresponding column electrode line of the matrix for each column. Also, the cathode electrodes are connected to a cathode driver IC 3 through the corresponding row electrode line of the matrix for each row.
The anode driver IC 2 includes switching elements Sa1 to Sam, constant current driving circuits CCg, and pull-down resistors Ra corresponding to the column electrode lines, respectively. Each of the switching elements Sa1 to Sam is controlled in accordance with an anode driver control signal that is supplied from a control circuit (not shown). Each of the constant current driving circuits CCg is a constant current driving circuit having an output stage transistor is for example a PMOS-FET, and supplies a constant current signal to the organic EL elements, which serve as loads, on the basis of a voltage Va that is supplied from an anode driver power circuit (not shown). The pull-down resistors Ra are connected to a ground.
On the other hand, the cathode driver IC 3 includes switching elements Sc1 to Scn, pull-up resistors Rc, and pull-down resistors Rg corresponding to the row electrode lines, respectively. Each of the switching elements Sc1 to Scn is controlled in accordance with a cathode driver control signal that is supplied from the control circuit. The pull-up resistors Rc are connected to a supply line of a voltage Vc that is supplied from a cathode driver power circuit (not shown), and the pull-down resistors Rg are connected to a ground.
The operation of the circuit shown in FIG. 1 will be described below. First, in a reset period in synchronization with a line synchronization pulse included in the anode driver control signal, all the switching elements of the anode driver IC 2 and the cathode driver IC 3 are switched to the pull-down resistor side so as to uniform residual charges in all parasitic capacitances.
After that, in a light emission period, the switching element corresponding to a scanned line (row) in the cathode driver IC 3 is selected and is maintained in the selection state of the pull-down resistor side. On the other hand, the other switching elements of the non-scan lines are switched to the pull-up resistor Rc side. Also, in the anode driver IC 2, any switching elements corresponding to EL elements to be driven are switched in accordance with the anode driver control signal supplied from the control circuit, the constant current circuits CCg are connected to the column electrode lines, and the other column electrode lines corresponding to non-light emission elements are connected to the ground via the pull-down resistors Ra.
In the circuit shown in FIG. 1, more specifically, the second row is selected as the scanned line and grounded through the pull-down resistor Rg, and the first column and the m-th column electrode lines are connected to the constant current circuits CCg, in correspondence with the elements that are to be light-emitted. Thus, a drive current is allowed to flow from the anode to the cathode of each of the organic EL elements at the points of intersection of these column electrode lines and the row electrode line, and the organic EL elements shown in white in the organic EL display panel of FIG. 1 are light-emitted.
As described above, in conventional display panel driving devices, it is necessary to provide the constant current driving circuit CCg for each column in the anode driver IC 2. As a result, the structure of the anode driver IC becomes complex, making it difficult to downsize the anode driver IC and to reduce the cost. Also, since a current is constantly supplied from the constant current driving circuit to the column electrode line corresponding to an EL element to be driven for light emission, there is a problem in that the overall power consumption of the IC increases.